animal kingdom chapter 4 - NCERT

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46BIOLOGYWhen you look around, you will observe different animals with different

structures and forms. As over a million species of animals have been described till now, the need for classification becomes all the more important. The classification also helps in assigning a systematic posit ion to newly described species.

4.1BASIS OF CLASSIFICATION

Inspite of differences in structure and form of different animals, there are fundamental features common to various individuals in relation to the arrangement of cells, body symmetry, nature of coelom, patterns of digestive, circulatory or reproductive systems. These features are used as the basis of animal classification and some of them are discussed her e.

4.1.1Levels of Organisation

Though all members of Animalia are multicellular, all of them do not exhibit the same pattern of organisation of cells. For example, in spong es, the cells are arranged as loose cell aggregates, i.e., they exhibit cellular level of organisation. Some division of labour (activities) occur among the cells. In coelenterates, the arrangement of cells is more complex. H ere the cells performing the same function are arranged into tissues, hence is called tissue level of organisation. A still higher level of organisation, i.e., organ level is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialise d for a particular function. In animals like Annelids, Arthropods, Mollusc s,ANIMAL KINGDOM

CHAPTER 4

4.1Basis of

Classification

4.2Classification ofAnimals

ANIMAL KINGDOM4747ANIMAL KINGDOM

Echinoderms and Chordates, organs have

associated to form functional systems, each system concerned with a specific physiological function. This pattern is called organ system level of organisation. Organ systems in different groups of animals exhibit various patterns of complexities. For example, the digestive system in Platyhelminthes has only a single opening to the outside of the body that serves as both mouth and anus, and is hence called incomplete. A complete digestive system has two openings, mouth and anus. Similarly, the circulatory system may be of two types: (i) open type in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it and (ii) closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries).

4.1.2Symmetry

Animals can be categorised on the basis of their

symmetry. Sponges are mostly asymmetrical, i.e., any plane that passes through the centre does not divide them into equal halves. When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry.

Coelenterates, ctenophores and echinoderms

have this kind of body plan (Figure 4.1a).

Animals like annelids, arthropods, etc., where

the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry (Figure 4.1b).

4.1.3Diploblastic and Triploblastic

Organisation

Animals in which the cells are arranged in two

embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, e.g., coelenterates. An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm (Figure 4.2a).Figure 4.2Showing germinal layers : (a) Diploblastic (b) Triploblastic (a)(b)

EctodermMesoglea

Endoderm

MesodermFigure 4.1 (b) Bilateral symmetryFigure 4.1 (a) Radial symmetry

48BIOLOGY

4.1.4 Coelom

Presence or absence of a cavity between the body

wall and the gut wall is very important in classification. The body cavity, which is lined by mesoderm is called coelom. Animals possessing coelom are called coelomates, e.g., annelids, molluscs, arthropods, echinoderms, hemichordates and chordates (Figure 4.3a). In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates, e.g., aschelminthes (Figure 4.3b). The animals in which the body cavity is absent are called acoelomates, e.g., platyhelminthes (Figure 4.3c).Figure 4.3Diagrammatic sectional view of : (a) Coelomate (b) Pseudocoelomate (c) AcoelomateThose animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals (platyhelminthes to chordates, Figure 4.2b).

4.1.5Segmentation

In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs. For example, in earthworm, the body shows this pattern called metameric segmentation and the phenomenon is known as metamerism.

4.1.6 Notochord

Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals. Animals with notochord are called chordates and those animals which do not form this structure are called non-chordates, e.g., porifera to echinoderms.

4.2CLASSIFICATION OF ANIMALS

The broad classification of Animalia based on common fundamental features as mentioned in the preceding sections is given in Figure 4.4.

ANIMAL KINGDOM5353ANIMAL KINGDOM

4.2.7Phylum - Arthropoda

This is the largest phylum of Animalia which

includes insects. Over two-thirds of all named species on earth are arthropods (Figure 4.12).

They have organ-system level of organisation.

They are bilaterally symmetrical, triploblastic,

segmented and coelomate animals. The body of arthropods is covered by chitinous exoskeleton. The body consists of head, thorax and abdomen. They have jointed appendages (arthros-joint, poda-appendages). Respiratory organs are gills, book gills, book lungs or tracheal system. Circulatory system is of open type. Sensory organs like antennae, eyes (compound and simple), statocysts or balance organs are present. Excretion takes place through malpighian tubules. They are mostly dioecious. Fertilisation is usually internal.

They are mostly oviparous. Development may

be direct or indirect.

Examples: Economically important insects -

Apis (Honey bee), Bombyx (Silkworm), Laccifer

(Lac insect)

Vectors - Anopheles, Culex and Aedes

(Mosquitoes)

Gregarious pest - Locusta (Locust)

Living fossil -

Limulus (King crab).

4.2.8Phylum - Mollusca

This is the second largest animal phylum

(Figure 4.13). Molluscs are terrestrial or aquatic (marine or fresh water) having an organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals. Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and visceral hump. A soft and spongy layer of skin forms a mantle over the visceral hump. The space between the hump and the mantle is called the mantle cavity in which feather like gills are present. They have respiratory and excretory functions. The anterior head region has sensory tentacles. The mouth contains a file-like rasping organ for feeding, called radula.Figure 4.12Examples of Arthropoda : (a) Locust (b) Butterfly (c) Scorpion (d) Prawn (a) (c) (b) (d) Figure 4.13Examples of Mollusca :(a) Pila (b) Octopus(b)(a)

54BIOLOGY

They are usually dioecious and oviparous with indirect development.

Examples:

Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea- hare), Dentalium (Tusk shell) and Chaetopleura (Chiton).

4.2.9Phylum - Echinodermata

These animals have an endoskeleton of calcareous ossicles and, hence, the name Echinodermata (Spiny bodied, Figure

4.14). All are marine with organ-system level of organisation.

The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical. They are triploblastic and coelomate animals. Digestive system is complete with mouth on the lower (ventral) side and anus on the upper (dorsal) side. The most distinctive feature of echinoderms is the presence of water vascular system which helps in locomotion, capture and transport of food and respiration. An excretory system is absent. Sexes are separate. Reproduction is sexual. Fertilisation is usually external.

Development is indirect with free-swimming larva.

Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily),

Cucumaria

(Sea cucumber) and Ophiura (Brittle star).

4.2.10Phylum - Hemichordata

Hemichordata was earlier considered as a sub-phylum under phylum Chordata. But now it is placed as a separate phylum under non-chordata. This phylum consists of a small group of worm-like marine animals with organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals. The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk (Figure 4.15). Circulatory system is of open type. Respiration takes place through gills. Excretory organ is proboscis gland. Sexes are separate. Fertilisation is external. Development is indirect.

Examples:

Balanoglossus and Saccoglossus.

4.2.11Phylum - Chordata

Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsalFigure 4.14Examples of

Echinodermata :

(a) Asterias (b) Ophiura(a) (b)Figure 4.15 Balanoglossus

ANIMAL KINGDOM5555ANIMAL KINGDOM

hollow nerve cord and paired pharyngeal gill slits (Figure 4.16). These are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation. They possess a post anal tail and a closed circulatory system.

Table 4.1 presents a comparison of salient

features of chordates and non-chordates.

Phylum Chordata is divided into three

subphyla: Urochordata or Tunicata,

Cephalochordata and Vertebrata.

Subphyla Urochordata and

Cephalochordata are often referred to as

protochordates (Figure 4.17) and are exclusively marine. In Urochordata, notochord is present only in larval tail, while in

Cephalochordata, it extends from head to tail

region and is persistent throughout their life.

Examples: Urochordata - Ascidia, Salpa,

Doliolum; Cephalochordata - Branchiostoma

(Amphioxus or Lancelet).

The members of subphylum Vertebrata

possess notochord during the embryonic period. The notochord is replaced by a cartilaginous or bony vertebral column in the adult. Thus all vertebrates are chordates but all chordates are not vertebrates. Besides the basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation and paired appendages which may be fins or limbs.

Nerve cordNotochord

Post-anal part

Gill slitsFigure 4.16 Chordata characteristics

Figure 4.17 AscidiaTABLE 4.1 Comparison of Chordates and Non-chordates

S.No.ChordatesNon-chordates

1.Notochord present.Notochord absent.

2.Central nervous system is dorsal,Central nervous system is ventral, solid

hollow and single.and double.

3.Pharynx perforated by gill slits.Gill slits are absent.

4.Heart is ventral.Heart is dorsal (if present).

5.A post-anal part (tail) is present.Post-anal tail is absent.

56BIOLOGYFigure 4.18 A jawless vertebrate - PetromyzonFigure 4.19Example of Cartilaginous fishes :

(a) Scoliodon (b) Pristis(a) (b)4.2.11.1Class - Cyclostomata

All living members of the class Cyclostomata are

ectoparasites on some fishes. They have an elongated body bearing 6-15 pairs of gill slits for respiration. Cyclostomes have a sucking and circular mouth without jaws (Fig. 4.18). Their body is devoid of scales and paired fins.

Cranium and vertebral column are

cartilaginous. Circulation is of closed type.

Cyclostomes are marine but migrate for

spawning to fresh water. After spawning, within a few days, they die. Their larvae, after metamorphosis, return to the ocean.

Examples: Petromyzon (Lamprey) and Myxine

(Hagfish).

4.2.11.2Class - Chondrichthyes

They are marine animals with streamlined body

and have cartilaginous endoskeleton (Figure 4.19). Mouth is located ventrally.

Notochord

is persistent throughout life. Gill slits are separate and without operculum (gill cover). The skin is tough, containing minute placoid scales. Teeth are modified placoid scales which are backwardly directed. Their jaws are very powerful. These animals are predaceous. Due to the absence of air bladder, they have to swim constantly to avoid sinking.Vertebrata

Division

Agnatha

(lacks jaw) Class

1. CyclostomataGnathostomata

(bears jaw)

Super Class

Pisces

(bear fins)Tetrapoda (bear limbs) Class

1. Amphibia

2. Reptilia

3. Aves

4. MammalsClass

1. Chondrichthyes

2. OsteichthyesThe subphylum Vertebrata is further divided as follows:

ANIMAL KINGDOM5757ANIMAL KINGDOM

Heart is two-chambered (one auricle and one ventricle). Some of them have electric organs (e.g., Torpedo) and some possess poison sting (e.g., Trygon). They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature. Sexes are separate. In males pelvic fins bear claspers. They have internal fertilisation and many of them are viviparous. Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).

4.2.11.3Class - Osteichthyes

It includes both marine and fresh water fishes with bony endoskeleton. Their body is streamlined. Mouth is mostly terminal (Figure 4.20). They have four pairs of gills which are covered by an operculum on each side. Skin is covered with cycloid/ctenoid scales. Air bladder is present which regulates buoyancy. Heart is two- chambered (one auricle and one ventricle). They are cold-blooded animals. Sexes are separate. Fertilisation is usually external. They are mostly oviparous and development is direct.

Examples: Marine - Exocoetus (Flying fish),

Hippocampus

(Sea horse); Freshwater - Labeo (Rohu),

Catla (Katla), Clarias (Magur); Aquarium - Betta

(Fighting fish), Pterophyllum (Angel fish).

4.2.11.4Class - Amphibia

As the name indicates (

Gr., Amphi

: dual, bios, life), amphibians can live in aquatic as well as terrestrial habitats (Figure 4.21). Most of them have two pairs of limbs. Body is divisible into head and trunk. Tail may be present in some. The amphibian skin is moist (without scales). The eyes have eyelids. A tympanum represents the ear. Alimentary canal, urinary and reproductive tracts open into a common chamber called cloaca which opens to the exterior. Respiration is by gills, lungs and through skin. The heart is three- chambered (two auricles and one ventricle). These are cold-blooded animals. Sexes are separate. Fertilisation is external. They are oviparous and development is indirect. Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog),

Salamandra (Salamander), Ichthyophis (Limbless

amphibia).Figure 4.21Examples of Amphibia : (a) Salamandra (b) Rana(a) (b)Figure 4.20Examples of Bony fishes : (a) Hippocampus (b) Catla(b) (a)

58BIOLOGY

4.2.11.5Class - Reptilia

The class name refers to their creeping or crawling mode of locomotion (Latin, repere or reptum, to creep or crawl). They are mostly terrestrial animals and their body is covered by dry and cornified skin, epidermal scales or scutes (Fig. 4.22). They do not have external ear openings. Tympanum represents ear. Limbs, when present, are two pairs. Heart is usually three-chambered, but four-chambered in crocodiles. Reptiles are poikilotherms. Snakes and lizards shed their scales as skin cast. Sexes are separate. Fertilisation is internal. They are oviparous and developm ent is direct. Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator).

Hemidactylus

(Wall lizard), Poisonous snakes - Naja (Cobra), Bangarus (Krait),

Vipera (Viper).

4.2.11.6Class - Aves

The characteristic features of Aves (birds) are the presence of feathers and most of them can fly except flightless birds (e.g., Ostrich). They possess beak (Figure 4.23). The forelimbs are modified into wings. The hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches. Skin is dry without glands except the oil gland at th e base of the tail. Endoskeleton is fully ossified (bony) and the long b ones are hollow with air cavities (pneumatic). The digestive tract of birds has additional chambers, the crop and gizzard. Heart is completely four- chambered. They are warm-blooded ( homoiothermous) animals, i.e., they are able to maintain a constant body temperature. Respiration is by Figure 4.22 Reptiles: (a) Chameleon (b) Crocodilus (c) Chelone (d) Naja(b)(c)(a)(d)

ANIMAL KINGDOM5959ANIMAL KINGDOM

lungs. Air sacs connected to lungs supplement respiration. Sexes are separate. Fertilisation is internal. They are oviparous and development i s direct. Examples : Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

4.2.11.7Class - Mammalia

They are found in a variety of habitats - polar ice caps, deserts, mo untains, forests, grasslands and dark caves. Some of them have adapted to fly or live in water. The most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which the young ones are nourished. They have two pairs of limbs, adapted for walking, runnin g, climbing, burrowing, swimming or flying (Figure 4.24). The skin of(a) Figure 4.23 Some birds : (a) Neophron (b) Struthio (c) Psittacula (d) Pavo(b) (c)(d)Figure 4.24 Some mammals : (a) Ornithorhynchus (b) Macropus (c) Pteropus (d) Balaenoptera(a) (b)(c) (d)

60BIOLOGY

mammals is unique in possessing hair . External ears or pinnae are present. Different types of teeth are present in the jaw. Heart is four- chambered. They are homoiothermous. Respiration is by lungs. Sexes are separate and fertilisation is internal. They are viviparous with few exceptions and development is direct. Examples: Oviparous-Ornithorhynchus (Platypus); Viviparous -

Macropus

(Kangaroo),

Pteropus (Flying fox), Camelus (Camel), Macaca

(Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale),

Panthera tigris (Tiger), Panthera leo (Lion).

The salient distinguishing features of all phyla under animal kingdom is comprehensively given in the Table 4.2.Level of

Organi-

sation

Cellular

Tissue

Tissue

Organ &

Organ-

system

Organ-

system

Organ-

system

Organ-

system

Organ-

system

Organ-

system

Organ-

system

Organ-

systemSymme-try

Various

Radial

Radial

Bilateral

Bilateral

Bilateral

Bilateral

Bilateral

Radial

Bilateral

BilateralCoelom

Absent

Absent

Absent

Absent

Pseudo

coelo- mate

Coelo-

mate

Coelo-

mate

Coelo-

mate

Coelo-

mate

Coelo-

mate

Coelo-

mateSegmen-tation

Absent

Absent

Absent

Absent

Absent

Present

Present

Absent

Absent

Absent

PresentDigestive

System

Absent

Incomplete

Incomplete

Incomplete

Complete

Complete

Complete

Complete

Complete

Complete

CompleteCircu-

latory

System

Absent

Absent

Absent

Absent

Absent

Present

Present

Present

Present

Present

PresentRespi-

ratory

System

Absent

Absent

Absent

Absent

Absent

Absent

Present

Present

Present

Present

PresentDistinctive

Features

Body with pores

and canals in walls.

Cnidoblasts

present.

Comb plates for

locomotion.

Flat body, suckers.

Often worm-

shaped, elongated.

Body segment-

ation like rings.

Exoskeleton of cu-

ticle, jointed ap- pendages.

External skeleton

of shell usually present.

Water vascular

system, radial symmetry.

Worm-like with

proboscis, collar and trunk.

Notochord, dorsal

hollow nerve cord, gill slits with limbs or fins.Phylum

Porifera

Coelenterata

(Cnidaria)

Ctenophora

Platyhelm-

inthes

Aschelmin-

thes

Annelida

Arthropoda

Mollusca

Echino-

dermata Hemi- chordata ChordataTABLE 4.2 Salient Features of Different Phyla in the Animal Kingdom

ANIMAL KINGDOM6161ANIMAL KINGDOMSUMMARY

The basic fundamental features such as level of organisation, symmetry, cell organisation, coelom, segmentation, notochord, etc., have enabled us to broadly classify the animal kingdom. Besides the fundamental features, there are many other distinctive characters which are specific for each phyla or class. Porifera includes multicellular animals which exhibit cellular level of organisation and have characteristic flagellated choanocytes. The coelen terates have tentacles and bear cnidoblasts. They are mostly aquatic, sessile or free-floating. The ctenophores are marine animals with comb plates. The platyhelminths have flat body and exhibit bilateral symmetry. The parasitic forms show disti nct suckers and hooks. Aschelminthes are pseudocoelomates and include parasitic as well as non-parasitic round worms. Annelids are metamerically segmented animals with a true coelom. The arthropods are the most abundant group of animals characterised by the p resence of jointed appendages. The molluscs have a soft body surrounded by an ex ternal calcareous shell. The body is covered with external skeleton made of chi tin. The echinoderms possess a spiny skin. Their most distinctive feature is the presence of water vascular system. The hemichordates are a small group of worm-li ke marine animals. They have a cylindrical body with proboscis, collar and trunk. Phylum Chordata includes animals which possess a notochord either throughout or during early embryonic life. Other common features observe d in the chordates are the dorsal, hollow nerve cord and paired pharyngeal gi ll slits. Some of the vertebrates do not possess jaws (Agnatha) whereas most of them possess jaws (Gnathostomata). Agnatha is represented by the class, Cyclostomat a. They are the most primitive chordates and are ectoparasites on fishes. Gnatho stomata has two super classes, Pisces and Tetrapoda. Classes Chondrichthyes and Osteichthyes bear fins for locomotion and are grouped under Pisces. The Chondrichthyes are fishes with cartilaginous endoskeleton and are marine . Classes, Amphibia, Reptilia, Aves and Mammalia have two pairs of limbs and are th us grouped under Tetrapoda. The amphibians have adapted to live both on lan d and water. Reptiles are characterised by the presence of dry and cornified s kin. Limbs are absent in snakes. Fishes, amphibians and reptiles are poikilothermou s (cold- blooded). Aves are warm-blooded animals with feathers on their bodies a nd forelimbs modified into wings for flying. Hind limbs are adapted for wal king, swimming, perching or clasping. The unique features of mammals are the p resence of mammary glands and hairs on the skin. They commonly exhibit viviparit y.

62BIOLOGY

EXERCISES

1.What are the difficulties that you would face in classification of anima

ls, if common fundamental features are not taken into account?

2.If you are given a specimen, what are the steps that you would follow to

classifyit?

3.How useful is the study of the nature of body cavity and coelom in the

classification of animals?

4.Distinguish between intracellular and extracellular digestion?

5.What is the difference between direct and indirect development?

6.What are the peculiar features that you find in parasitic platyhelminthe

s?

7.What are the reasons that you can think of for the arthropods to constit

ute the largest group of the animal kingdom?

8.Water vascular system is the characteristic of which group of the follow

ing: (a) Porifera (b) Ctenophora (c) Echinodermata (d) Chordata

9."All vertebrates are chordates but all chordates are not vertebrates"

. Justify the statement.

10.How important is the presence of air bladder in Pisces?

11.What are the modifications that are observed in birds that help them fly

?

12.Could the number of eggs or young ones produced by an oviparous and vivi

parous mother be equal? Why?

13.Segmentation in the body is first observed in which of the following:

(a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Arthro poda

14.Match the following:

(a)Operculum(i)Ctenophora (b)Parapodia(ii)Mollusca (c)Scales(iii)Porifera (d)Comb plates(iv)Reptilia (e)Radula(v)Annelida (f)Hairs(vi)Cyclostomata and Chondrichthyes (g)Choanocytes(vii)Mammalia (h)Gill slits(viii)Osteichthyes

15.Prepare a list of some animals that are found parasitic on human beings.